Courtesy: NASA/Goddard Space Flight Center - Meandering around the planet like a rollicking roller coaster in the sky, the Northern Hemisphere's polar jet stream is a fast-moving belt of westerly winds that traverses the lower layers of the atmosphere. The jet is created by the convergence of cold air masses descending from the Arctic and rising warm air from the tropics. Deep troughs and steep ridges emerge as the denser cold air sinks and deflects warm air regions north, giving the jet stream its wavy appearance. This pattern propagates across the mid-latitudes of North America, Europe and Asia, as pockets of cold air sporadically creep down from the Arctic—creating contrasting waves and flows that accelerate eastward due to Earth's rotation. The visualization below uses weather and climate observations from NASA's MERRA dataset to model 30 days of the jet stream's whirling journey over North America.